The present invention relates to a device for receiving and transporting objects, preferably objects of flat shape, the device having at least one pair of parallel chains or belts situated opposite each other and driven in synchrony, which define a space between each other for transporting the objects therethrough, each chain or belt having at least one gripper and each gripper of a chain or belt facing a respective gripper of the other chain or belt, each gripper being mounted on the chain or belt so as to be pivotable about a pivot axis oriented transversely to the direction of movement of the chain or belt, each gripper having an extension in spaced relationship to the pivot axis, which extension may be moved along a guiding path of a guiding plate, each chain or belt being associated with a guiding path, and the respective guiding path having a path section sloped with respect to the conveying direction of the objects at the point where the objects are received, so that, when the chain or belt is moving, the extension of the gripper is deflected along the sloped path section and thus the gripper is imparted a rotational movement which counteracts the translational movement caused by the chain or belt.
DE 31 46 931 A1 discloses such a device intended for feeding bar material. There an endless driving traction member is arranged on a frame, which member carries a plurality of clamps for the bar material at its periphery, which clamps cooperate with guiding plates fixed on the frame while they are moving. Each clamp consists of a pair of double-arm levers arranged so as to be rotatable in a horizontal plane and spring-loaded in the direction opposite the direction of movement of the endless traction member. Self-locking profiles eccentric with respect to their rotational axes are formed on the lever arms facing each other and cooperate with the bar material during movement of the traction member; the opposing arms of the levers are adapted for cooperation with the guiding plates. However, because of the straight portions of the guiding plates the relative speed between the clamps and the bar material is not zero, which makes exact machining impossible and results in low processing speed.
DE 30 38 425 A1 teaches a device for conveying objects of flat shape (insulating glass panes), wherein the objects are held and conveyed by grippers arranged on conveying chains.
Furthermore, industrial production often encounters the problem that in the course of an automatic manufacturing operation objects transported along a transporting distance have to be stopped in order to subject them to a processing step. After this processing step has been completed the objects again have to be received by the transporting system and moved on to the next processing station.
A practical example for the above production operation can be found in the production of sacks. In the course of this, tubular material webs are continuously fed into a sack manufacturing installation from a drum. This continuous movement of the material webs has to be converted into an intermittent movement so that sack bodies of appropriate length may be cut off. The sack bodies lying still are then transported to further processing stations, like a sewing machine, a sack bottom folding and closing machine and a printing machine. In the course of this manufacturing process it will be necessary again and again to bring the sack body to a halt and to move it on again, changes of direction sometimes being necessary as well.
Two basic types of devices for receiving objects in a transport system and conveying them are known in the art. The first type is that of a discontinuously operating conveying belt. Here the object to be transported is clamped to the belt by suitable means while the belt is not moving, the belt subsequently being accelerated. In case of the production of sacks the sack body is conveniently attached to the belt by vacuum.
The fundamental disadvantage of this type is the stop-and-go operation, resulting in high wear of the individual components, high energy consumption and thus high operation expenses. Such a device requires frequent maintenance, and in addition is rather imprecise in its operation, which in case of the manufacturing of sacks results in high rejection rates, as exact positioning of the sack body, for instance at the sewing machine or the printing machine, is indispensable.
The second known type of a device for receiving and transporting objects is that with continuously moving arms, gripping the object with gripping claws and preferably transmitting it to a conveying belt. Thus in the case of the production of sacks it is not necessary to move significant masses, which results in less energy consumption of the device. What is required, however, is sophisticated electronic control, and because of the inevitable component and fitting tolerances, as well as because of wear, this machine has to be readjusted quite frequently. The throughput rates of several dozen sacks per minute, which are typical for the production of sacks, make it necessary for the gripping claws to engage a sack body in coordinated fashion within a fraction of a second, which is difficult to achieve and thus frequently results in the sack body being pulled into the conveying system at an angle, which in turn results in high rejection rates.
Thus it would be desirable to be able to provide a device for receiving and transporting objects, in particular objects of flat shape, as for instance sack bodies, which is reliable, works highly accurately, hardly needs any maintenance and readjustment, and in addition permits high processing speeds.
This aim is achieved according to the present invention by developing a device of the above kind in such a way that the sloped path section of the guiding path of each guiding plate is curved at the point of receipt. As a result, that component of the circular speed given to the engagement end of the gripper which is directed against the transportation speed of the objects is not constant but subject to acceleration, bringing the relative movement between the grippers engagement end and the object close to zero at the time of receipt.
This device according to the invention is very reliable as it comprises no parts which would require complicated adjustment or tend to be misadjusted during operation. The guiding plates may be made of massive metal plates, so that wear of the guiding paths by the sliding movement of the extensions of the grippers is negligible. As the guiding plates are fixedly mounted on the device, the position of the guiding paths always remains the same. In order to transport objects of a different size the guiding plates are simply exchanged. Thus the device may be operated by personnel without much training, which greatly reduces the operation expenses. As compared to a translational movement directed towards the object to be gripped, the rotational movement carried out by the grippers has the major advantage that, as seen in direction of transportation, the relative movement between the engagement end of the grippers and the object approaches zero at the time of receipt, ensuring synchronous gripping by the grippers even at high transportation speeds.
In order to ensure uniform force of engagement by the grippers all along the distance of transportation of the objects the guiding path of the respective guiding plate runs parallel to the direction of transportation of the objects after the point of receipt.
At a point where the objects are discharged the guiding path of the respective guiding plate conveniently has a (preferably curved) path section sloped in opposite direction to the inclination of the guiding path at the point of receipt.
According to one embodiment of the invention the grippers are mounted in bearing shoes on the chain or belt.
In order to make the distance to be travelled by the grippers for gripping the objects at the point of receipt as short as possible, the guiding path of each guiding plate runs at a distance to the chain or belt in a feeding zone upstream of the point of receipt, where the respective gripper is pre-deflected close to the object by its extension.
Conveniently the grippers are biased to an inoperative position by spring means, where an engagement end of the gripper is at a distance from the object to be received.
A favorable embodiment of the invention is characterized in that the extension of each gripper may be moved along between two preferably nearly parallel guiding paths facing each other. Thus the gripper may be deflected towards the object as well as away therefrom, the deflection forces exerted on the gripper being much higher than the restoring forces exerted by the spring means, it being possible, however, to additionally use spring means in order to bias the grippers to a defined position outside the guiding paths, i.e. where the chain is returning.
In order to improve gripping of the objects the grippers advantageously have engagement jaws of elastomeric material.
In order to minimize friction between the extension for deflecting the gripper and the guiding path of the guiding plate, the extension may be provided with a freely rotatable roller.
In order to achieve a desired leverage the extension for deflecting the gripper is positioned on a cantilevered arm of the gripper.
In order to prevent the chain or belt from sagging along the transportation distance of the objects, which would result in reduced engagement force of the grippers, a support structure may be provided close to the chain or belt, where the chain or belt and the bearing shoe, respectively, of the grippers may rest.
The invention will now be explained in more detail by way of example with reference to the attached drawings.